1. Field of the Invention
The present invention is generally directed toward a process to create acrylic acid and methyl acrylate esters. More specifically, the present invention is directed toward a process to create technical grade acrylic acid or/and methyl acrylate ester from renewable resources.
For purposes of this invention, the term green technical grade acrylic acid or green acrylic acid refers to technical grade acrylic acid derived from renewable resources.
2. Background
The acrylic acid market is measured by the production of crude acrylic acid. Crude acrylic acid (also known as technical grade acrylic acid) is not an item of commerce. However, it is either further purified into glacial acrylic acid or converted into Acrylate esters. The market is equally split between glacial and ester production (i.e. 50% of the crude goes to glacial and 50% goes to esters). The worldwide capacity for crude acrylic acid has been estimated at over 9 billion pounds per year.
All current production of crude acrylic acid is via a two stage air oxidation of propylene. In the first stage propylene is oxidized to acrolein using an expensive Bi/Mo based mixed metal oxide catalyst. In the second stage the acrolein is oxidized to acrylic acid using an expensive Bi/V based mixed metal oxide catalyst. Both oxidation steps are conducted at high temperature (320° C. and 280° C., respectively) in very expensive shell and tube reactors using molten salt heat exchange fluids.
The hot gases exiting the second reactor are rapidly cooled and the non-condensibles are separated from the condensed aqueous acrylic acid solution in the absorber. The concentration of the acrylic acid in this aqueous solution depends on the technology employed. One technology uses steam injection into the reactors to control flammability and the other uses recycle gas injection instead of steam. Steam injection can lead to an aqueous acrylic acid solution as low as ˜20% while recycle gas injection can produce an aqueous acrylic acid as high as 70% leaving the absorber.
This aqueous acrylic acid is then subjected to a complicated purification system consisting of several towers to produce crude acrylic acid (technical grade). In the first tower water is removed. If steam was used as the diluent in the reactors the water is removed via extraction and azeotropic distillation is used if recycle gas was employed. In both cases the dewatered acrylic acid is then subjected to multiple vacuum distillations to remove both light and heavy by-products. The final product from these distillation steps is technical grade acrylic acid (>99% purity).
The capital cost for a crude acrylic acid unit is very high. Furthermore, the high raw material cost of propylene make it vulnerable to a new technology for some of the future Acrylic acid production units.
Currently, there is no commercially viable micro-organism which can directly produce acrylic acid via fermentation. However, there are known micro-organisms which can produce specific hydroxypropionic acids (acrylic acid precursors) via glucose fermentation. There are two configurational isomers of hydroxypropionic acid. The alpha isomer is commonly known as lactic acid and the beta isomer is better known a 3-hydroxypropionic acid (3HPA). Lactic acid has been produced on a commercial scale via fermentation for over one hundred years while 3HPA is not yet commercially available.
Both isomers undergo acid catalyzed dehydration yielding acrylic acid, see Chemical Reaction 1 as illustrated in FIG. 1.
However, the two isomers yield different amounts of acrylic acid. The beta isomer (3HPA) dehydrates in near quantitative yields while the alpha isomer (lactic acid) only realizes ˜55% yield. These dehydration efficiencies are essentially the same for both the free acids and the corresponding lactate esters. The reason for this difference in selectivity to acrylic acid is most likely related to the location of the intermediate carbocation. Lactic acid proceeds through a carbocation alpha to the carbonyl (which can readily undergo decomposition) and 3HPA proceeds through a carbocation beta to the carbonyl (i.e. the positive charge is removed from the carbonyl and can only readily eliminate a proton forming acrylic acid).
While the dehydration of lactic acid to acrylic acid has been studied for over 50 years, the yield remains poor. This poor dehydration efficiency is also observed for lactate esters. However, it has been shown that the acetylated product of lactic acid (2-acetoxypropionic acid) readily undergoes pyrolysis to acrylic acid in ˜95% yields, see Chemical Reaction 2 as illustrated in FIG. 2. High yields have also been reported for the pyrolysis of methyl 2-acetoxypropionate.
This pyrolysis reaction is a cyclic elimination of acetic acid and goes in high yields because it does not proceed through the carbocation intermediate associated with the dehydration of lactic acid. Obviously lactic acid could be converted into this acetoxy derivative and then pyrolyzed to produce acrylic acid in high yields. The problem with this route is that the acetoxy derivative would be typically made by reaction of lactic acid with either acetic anhydride or ketene. The recovered acetic acid could be converted back to anhydride or ketene using a ketene furnace, but a ketene furnace is very expensive. Furthermore, the lactic acid is only available as an aqueous solution so excess ketene or anhydride would be consumed by the water present in the aqueous lactic acid further decreasing the economics of this route. To utilize this route via the acetoxy derivative one must be able to prepare it directly from acetic acid.
Thus, there is a need for a more effective and efficient process to create acrylic acid and methyl acrylate ester.